Father was showing a machine he invented to son.Father said " Son, what do you think of my invention?".He took pig,put it in the machine and at the other end out came sausages.Son said "Dad,this invention no good.You should invent one where you put in sausages and out comes a pig"Father,fuming but kept his cool and said"Son,this machine no need to invent.Exist already - your mom.I put in the sausage,you're the pig"